Steel coil springs in the wheel assemblies of automobiles and other vehicles are subjected to very corrosive conditions. Conventional steel springs were expected to rust, and to allow for such rusting, conventional coil springs were formed of appropriately thick steel.
To improve the steering and ride control of automobiles, it is desirable to move the wheels outward, toward the corners of the vehicle. This increases torsional stress on the automobile body structure which must be nullified using a stronger frame assembly or weight reduction of the suspension components moved toward the corners. Reduced diameter, high tensile steel, coil springs weigh less than conventional automotive suspension coil springs, so they offer means to reduce the weight of these components. Super high tensile steel offers the promise of further weight reduction.
Herein, high tensile steel is defined as having MPa) (megapascal (N/m2) ranging from 1800 Mpa to 2100 Mpa; this includes super high tensile steel from 1950 Mpa to 2100 Mpa or above.
High tensile steel, coil springs are scratch and notch sensitive, so they require protection from impact damage caused by flying stones and gravel encountered during driving on paved or unpaved roads. Also, to maintain desired metallurgical properties and prevent premature flex damage failure, the high tensile steel cannot be heated beyond 325° F. (163° C.).
Conventional “E” coat primers and/or epoxy powder coatings used on strut type automotive suspension springs are unacceptable at inhibiting flying stone damage as determined by low temperature gravelometer testing followed by accelerated scab corrosion testing of high tensile springs. Neither layer applied alone will provide the chip and corrosion protection required.
A current approach to providing corrosion and chip resistance to high tensile steel is described in U.S. patent application Ser. No. 08/728,237 filed 8 Oct. 1996, the teachings of which are incorporated herein by reference. This patent application describes a dual layer coating including a zinc-rich, thermoset, epoxy base coat which provides corrosion-resistance and a thermoplastic outer coating which provides chip-resistance. The particular thermoplastic resin taught in U.S. patent application Ser. No. 08/728,237 is an ethylene/acrylic acid copolymer, a relatively expensive material. Polyolefins can be substituted for ethylene/acrylic acid copolymer, and although polyolefins are less expensive materials, thicker layers are required to provide the requisite chip-resistance. Also, processing of thermoplastics is expensive. A general disadvantage of such a dual layer coating is that two separate coating operations are required and two separate heating cycles are required, the first to fuse and cure, at least in part, the epoxy resin of the base coat and a second to fuse the thermoplastic resin of the outer coat plus complete curing of the epoxy base coat. Both the material of the base coat and the outer coat must be cycled at a temperature below that whereat the steel would lose its high tensile strength; accordingly, the choice of suitable resin systems for the two layers is limited.
It is accordingly an object of the present invention to provide coatings for high tensile steel which are less expensive than the coatings currently in use and which can be processed with a single heat cycle.